I didn't do any of the indoor cycling classes at the convention I just finished - I can't ride those bikes because of the length of the crankarms - but I was in a quieter session next door to the cycling room when we overheard the presenter yelling "I am appalled that no one is doing push-ups!" This is where the instructors get their training...

WRT cadence, just from an outdoor and trainer/rollers POV, there's no such thing as "too fast" per se, but there's "poor form" (bouncing) and obviously also "inefficient." An instructor should know the difference... Without knowing what cadence the instructor is asking you to maintain - if it's anything between 85 and 105, then IMO it's definitely beneficial to maintain that for an entire class even if it might not be your preferred cadence on the road. The lower cadence at higher tension will imitate a long climb and build strength, a moderately high cadence will help smooth your pedal stroke, get you great cardio and build your inner thigh muscles.